The present invention relates to a negative pressure supply apparatus for supplying a negative pressure to a pneumatic booster of an automotive brake system, for example.
In general, an automotive brake system is provided with a pneumatic booster to increase braking force. The pneumatic booster uses the engine intake (negative) pressure as a negative pressure source. That is, the engine intake (negative) pressure is introduced into a constant-pressure chamber (negative pressure chamber) to produce a differential pressure between the intake pressure and the atmospheric pressure, thereby generating thrust in a power piston to assist the brake system with operating force.
In recent years, automotive engines have been improved to reduce pumping loss in order to meet the demand for lower fuel consumption. Accordingly, the engine intake (negative) pressure is tending to decrease. Consequently, the negative pressure supplied to the pneumatic booster is likely to become insufficient.
In view of the above-described problem, a conventional technique uses an electrically-driven rotary vacuum pump as a negative pressure source of a pneumatic booster, so that a sufficient negative pressure can be supplied to the pneumatic booster irrespective of the engine running condition, as disclosed, for example, in Japanese Patent Application Unexamined Publication (KOKAI) No. 2002-195178.
However, the vacuum pump disclosed in the above-described publication suffers from the following problems. The conventional technique uses a vane pump as a vacuum pump. The vane pump has a complicated structure and a high production cost and is difficult to reduce in size. It is also conceivable to use a reciprocating piston type vacuum pump having a simple structure. In the reciprocating piston type vacuum pump, however, the atmospheric pressure acts on the piston as back pressure. Therefore, if the pump is used to obtain a high degree of vacuum required for the pneumatic booster, the load variation increases, and it becomes difficult to perform a smooth operation.